1. Field of the Invention
This invention relates to photocurable organopolysiloxane compositions which are able to cure by application of actinic light, particularly UV light, to provide rubber elastomers which have good peelable protective properties during and after heating which will be hereinafter called peelability after heating. The composition is particularly useful as a masking material for printed wiring boards or temporary adhesives for precision parts. It will be noted that the term "peelability after heating" is intended to mean peelability of the composition from other materials after heating, for example, in a solder bath.
2. Description of the Prior Art
It is well known that organopolysiloxanes heated in the presence of organic peroxides provide silicone rubber elastomers which have good heat and cold resistances, weatherability and electric characteristics. It is also known that certain types of organosiloxanes are cured by irradiation of light in the presence of a photo-initiator.
For instance, Japanese Patent Publication No. 52-40334 (corresponding to U.S. Pat. No. 4,064,027) and Japanese Laid-open Patent Application No. 60-104158 describe photocurable organopolysiloxane compositions wherein vinyl group-containing polysiloxanes and mercapto group-containing polysiloxanes are cured through photo-radical addition reaction. However, the use of the mercapto group-containing polysiloxanes involves the problem of offensive odor and metal corrosion, thus placing a limit on the application of the compositions.
Japanese Patent Publication No. 53-36515 (corresponding to Great Britain Patent Specification No. 1323869) and Japanese Laid-open Patent Application No. 60-215009 (corresponding to U.S. Pat. No. 4,675,346) propose photocurable compositions which are comprised of acryl group-containing polysiloxanes and sensitizers therefor. For obtaining rubbery elastomers from these compositions, use of linear polymers with a high molecular weight is necessary. This leads to a relative reduction in amount of acryl groups provided at terminal ends of the polysiloxane, so that curability becomes poor. In addition, the composition undergoes little curing at the surface portion exposed to air because of the impediment of the curing with oxygen. To avoid this, polysiloxanes having a relatively large quantity of acryl groups are actually used. Such a composition undesirably provides a resin rather a rubbery elastomer.
We have already proposed in Japanese Patent Application No. 62-15414 a photocurable composition which comprises a novel acryloxyalkylsilanol compound, a linear polysiloxane wherein a plurality of acryl groups are introduced at terminal ends of the linear polymer, and a sensitizer. This composition provides a silicone rubber elastomer but the elastomer does not have a good peelability-after-heating property.